General information on the laying of roadway topcoats, asphalt intermediate layers, asphalt binder layers and/or asphalt sealing layers from asphalt mixtures is presented in “Technical delivery conditions for asphalt mixtures for the construction of public thoroughfare paving” (TL Asphalt StB 07, 2008 version). Classifications for grain sizes from stones to stone dust (fillers) are found in “Technical delivery conditions for stone sizes in road construction” (TL Stones-StB 04, 2007 version). There one also finds the designations for the different stone classifications.
Roadway topcoats, intermediate layers, binder layers and/or sealing layers made of asphalt are differentiated between two installation methods, rolled asphalt and mastic asphalt.
Rolled asphalt gets its name because it must in general be compressed by heavy rollers, with or without vibration, after installation and precompression. On the other hand, mastic asphalt can be poured, distributed, and installed in its final position with an installation machine (mastic asphalt screed or road paver), and thus need not be compressed. Due to its properties during installation, mastic asphalt is already prepared so that compression by heavy rollers is not required.
With rolled asphalt the mixture already contains the aggregate (stone chippings and sand) to create a textured road surface of the roadway topcoat that is rich in stone chippings causing it to have good gripping properties. Rolled asphalt must nonetheless be compressed by means of heavy rollers to the degree of compression required by rules and standards. Incomplete compression of rolled asphalt after installation is a primary cause of damage to roadway topcoats. It is often the case that installation errors occur due to unfavorable installation conditions, e.g., incomplete compression.
The technical shortcomings of rolled asphalt installation argue in favor of using mastic asphalt to lay roadway topcoats, intermediate layers, binder layers and/or sealing layers, especially for road construction. Roadway topcoats, intermediate layers, binder layers and/or sealing layers are thus all types of covering layers, intermediate layers, binder layers, and sealing layers encountered in road building, including situations in which the respective layers are not driven on in the narrower sense. It also includes all types of miscellaneous surfaces, even on bridges and in tunnels.
Mastic asphalt is a mixture of stone chippings, sand, filler, and binding agent. The filler is formed from the smallest grain sized powdered stone <0.063 mm (TL stone-StB 04, 2007 version), which is generally used as the binding agent in bitumen. This mixture is composed so that it can be poured and coated during installation, and can thus also be used as a mastic asphalt floor screed. No further compression is required after installation and finishing of the mastic asphalt layer.
Mixtures, and particularly mineral mixtures, general aggregate mixtures of class 0/8 (with a maximum grain size of 8 mm) or 0/5 (with a maximum grain size of 5 mm) or even 0/3 (with a maximum grain size of 3 mm) are appropriately used in road construction as a function of the surface properties. These mixtures are of special significance for roadway topcoats optimized for low noise properties. The grain size is important for determining the surface properties, tire gripping characteristics, rolling properties and noise produced by the roadway topcoat. They play a subordinate role with regard to stability, since this is primarily determined by the coating, which consists mainly of sand, filler, and binding agent.
Due to its favorable installation and flow properties, a roadway topcoat made of a mastic asphalt mixture would have a closed, smooth surface. This would not be well suited for driving. Stone chippings must therefore be provided for a roadway topcoat. In order to increase the roughness and tire gripping capability of the surface, it is conventional practice with mastic asphalt to scatter stone chippings or sand, which must be worked into the surface.
The application of scattered stone chippings with mastic asphalt requires the operational steps of heating, encasing with bitumen, transport and storage at the jobsite, scattering a specified quantity (12-15 kg/sqm), kneading with a rubber roller, rerolling with a smooth roller, sweeping, collecting, and transporting the excess scattered stone chippings away. Risks to tire gripping capability, noise production, and driving comfort also result from this sequence of procedures. If the mastic asphalt mixture has too much mortar on the surface, then the scattered stone chippings will sink into it. If too thin a coating is applied to the surface, then the subsequently scattered stone chippings will not adhere. This can also produce rough surfaces due to different installation thicknesses and excessively low temperatures, or even improper rolling.
Nonetheless, mastic asphalt is the most economical and most durable roadway topcoat made of asphalt for high traffic areas. The binding agent, namely bitumen, is protected from oxidation and subsequent embrittlement by the impermeability of mastic asphalt, and water is prevented from penetrating into the road structure.
More recently mastic asphalt mixtures have also been adapted in a modified procedure to make roadway topcoats. This mastic asphalt, which has improved noise qualities, is processed at 10 to 13 kg/sqm of scattered stone chippings (coarse aggregate) having special properties with respect to grain size and shape. These scattered stone chippings are not yet rolled. The installation of this type of mastic asphalt mixture is very difficult, however, since all installation parameters, such as installation temperature, absence of precipitation even on the day before the installation, proper temperature of the hot scattered stone chippings, must be maintained.